[Bier] Questions on draft-venaas-bier-mtud-01

[Eric C Rosen <[e-mail]>]

"The BIER MTU of an interface is the largest BIER encapsulated payload
   that can be sent out of the interface"

Please clarify whether "the BIER MTU of an interface" is:

- the size of the largest payload that can be BIER-encapsulated and then 
sent out the interface, or

- the size of the largest BIER packet (including BIER header and 
payload) that can be sent out of the interface.

The procedures in this draft don't seem to take account of scenarios 
such as those discussed in section 6.9 of RFC 8279, where a BFR may have 
to tunnel a packet to the next  BFR, by applying additional 
encapsulation before transmission.  Isn't it possible in these scenarios 
that the procedures of the draft will result in a perceived sub-domain 
MTU that is actually too  large?  A similar issue might arise if a 
"repair path" is used to get from one BFR to another, since that would 
increase the size of the encapsulation header.

Since the sub-domain MTU is determined by examining the IGP messages, 
how would a source host (that presumably does not participate in the 
IGP) learn what the sub-domain MTU is?  Or is it the intention that the 
BFIR do any necessary fragmentation?  The latter wouldn't work for IPv6 
though (except in the case where the source host is also the BFIR).

Another difficult scenario for this procedure is one where a packet has 
to traverse multiple BIER domains to reach its destinations.

Another question is whether this is really of any practical value. Is 
there a known use case?